Fix literal variable-name inclusion in rotation mixins

Stylus was getting confused by the attempted interpolation of the rotation
value in the `filter` property and rendering the the `filter` value with
a literal `{$rotation}` string in it.  Using string addition resolves the
value of the variable and generates a valid `filter` value.
Merge pull request #7 from justinlocsei/mixin-interpolation

Fix literal variable-name inclusion in rotation mixins